1. Field of the Invention
This invention relates to a method of and apparatus for carrying out image processing on an image signal obtained from a recording sheet on which a radiation image is recorded and particularly on an image signal representing a radiation image recorded using an irradiation field stop for limiting the irradiation field of radiations.
2. Description of the Related Art
Carrying out a suitable image processing on an image signal obtained by reading a recorded radiation image and reproducing a visible image on the basis of the processed image signal have been practiced in various fields, for instance, in a radiation image recording and reproducing system using stimulable phosphore sheets disclosed in our many patent applications.
When recording a radiation image on a recording sheet, it is often desired that portions of the object not related to diagnosis or the like be prevented from being exposed to radiations, thereby preventing adverse influence of radiations on the object. Further when the object portions not related to diagnosis or the like are exposed to radiations, the radiations are scattered by such portions and the quality of the radiation image is lowered by the scattered radiations. Therefore, an irradiation filed stop is often used in order to limit the irradiation field so that the radiations are irradiated only on the necessary part of the object.
In such a case, the read-out conditions and image processing conditions are determined on the basis of the image signal components corresponding to the picture elements in the irradiation field. The irradiation field can be recognized, for instance, in the following manner as disclosed in U.S. Pat. No. 4,967,079. That is, image signal components for picture elements on each of a plurality of radial linear segments joining a predetermined point in the irradiation field and a plurality of points on the edges of the recording sheet are read out, and a prospective edge point, which is considered to be on the edge of the irradiation field, is determined for each segment, and the irradiation field is recognized as a region which is circumscribed by a line passing through the prospective edge points.
The radiation image is reproduced, for instance, on a CRT as a visible image or recorded on a photographic film as a visible image by use of a laser printer or the like on the basis of an image signal obtained according to the read-out condition and the image processing conditions thus determined.
However in the case where the radiation image, which is the original of the visible image, is recorded by use of an irradiation field stop, the area of the reproduced image corresponding to the area outside the irradiation field is high in brightness (as reproduced on the CRT) or low in density (as recorded on the film) since the area outside the irradiation field is hardly exposed to radiations. Strong light impinging upon the eyes from the area outside the irradiation field makes the image in the irradiation field difficult to view no matter how high the quality of the image in the irradiation field is. In order to overcome such a problem, conventionally a light-shielding plate or the like is sometimes disposed on the CRT or the film to block light from the area outside the irradiation field.
However since the shape and/or size of the irradiation field differ according to the object, it is troublesome to change the position of the light-shielding plate by the shape and/or size of the irradiation field. Further it is very difficult to clearly separate the area inside the irradiation field from the area outside the irradiation field by the light-shielding plate, and the light-shielding plate sometimes covers up to the edge of the irradiation field and sometimes permits leak of light from the area outside the irradiation field. Thus, the light-shielding plate cannot satisfactorily overcome the aforesaid problem of difficulties in viewing the image in the irradiation field due to light from the area outside the irradiation field.
In the method of and apparatus for processing a radiation image disclosed in Japanese Unexamined Patent Publication No. 3(1991)-98174, the image signal components for picture elements outside the irradiation field are set to a low brightness (when a visible image is reproduced as a brightness distribution on a CRT or the like) or high density (when a visible image is reproduced as a density distribution on film or the like), generally to a minimum brightness or a maximum density, thereby obtaining a visible image free from strong light impinging upon the eyes from the area outside the irradiation field.
However when the area outside the irradiation field is set uniformly to a maximum density (or a minimum brightness), the density of the reproduced image sharply increased to the maximum density along the edge of the irradiation field, which is unnatural.
Further generally the irradiation field cannot be always recognized precisely and an area narrower than the actual irradiation field can be sometimes mistaken for the irradiation field. In such a case, a part of the radiation image which should be viewed for, for instance, diagnostic purposes can exist in the area determined to be outside the irradiation field. The image processing employed in the above identified Japanese patent publication results in a cut in such a necessary part of the image signal which is determined to be outside the irradiation field by mistake.
In the image processing disclosed in Japanese Unexamined Patent Publication No. 7(1995)-226882, the value of each picture element in the area outside the irradiation field is converted according to the following formula. EQU g(x,y)=a.multidot.f(x,y)+(1-a).multidot.f.sub.max
wherein f(x,y) and g(x,y) respectively represent the values of the picture element in a position (x,y) before and after the conversion, f.sub.max represents a maximum density, and a represents a coefficient which is 1 for picture elements within the irradiation field and is linearly reduced as the distance of the picture element from the edge of the irradiation field increases as shown in FIG. 4 for picture elements outside the irradiation field (0.ltoreq.a.ltoreq.1). In accordance with this processing, change in density near the edge of the irradiation field in the reproduced image can be smoothened to some extent and the reproduced image is free from strong light impinging upon the eyes when viewing the reproduced visible image without cutting a necessary part of the image signal which is determined to be outside the irradiation field by mistake.
However since the coefficient a is discontinuous on the edge of the irradiation field where a=1 and at portions outside the irradiation field where a=0, an artifact in the form of double lines appears in the reproduced image along the edge of the irradiation field, which adversely affects diagnosis through the reproduced image.